The process of applying atomized liquid coatings and adhesives generates potentially dangerous gaseous and particulate byproducts that are controlled or managed by confining them in an enclosure known as a spray booth and conveying them away from the process by entwining them in a moving air stream. This exhaust air stream typically passes through one or more stages of filtration to remove the particulates before the gaseous or vaporous byproducts are exhausted into the atmosphere. The volume of the exhaust air stream varies according to the size of the spray booth and may range, for example, from 3,000 cubic feet per minute (CFM) to more than 50,000 CFM. This equates to 30 to 150 lineal feet per minute within the spray booth in the direction of air flow. Provision may be made to replace the exhausted air volume.
Since most coating processes are vulnerable to quality rejects caused by dirt or other foreign airborne objects, prudent finishing process operators equip their facilities with air make-up unit. Air make-up units have a three fold function. First, they supply the process with the required replacement air. Second, they filter the replacement air. Third, they heat or condition it. An air make-up unit may be directly coupled to the spray booth or it may dump replacement air to the area surrounding the booth. Air make-up units are designed to include a heat source with sufficient thermal capacity to warm the volume of replacement air to the desired temperature on the anticipated coldest day. Typically, a fan or blower in the air make-up unit pushes or pulls the entire replacement air stream through a complex assembly that includes filters, dampers, and a heat source which is usually a gas burner.
Historically, environmental and worker health and safety regulations have empirically established minimum air velocities for spray booths. The overwhelming majority of installed spray booths are equipped with fixed speed exhaust fans or blowers. While exhaust air velocities change as the particulate filtration system loads, the pressure drop increase across the filtration system is usually limited to 0.5 inches of water column and produces a corresponding reduction in exhaust air volume in the range of 20%. Since the spray booth exhaust air volume is essentially fixed in any given installation, the volume of air the associated air make-up unit is required to provide also is fixed. Hence, the air make-up unit like the spray booth is usually equipped with a fixed speed fan or blower. In addition, the gas burner's operating efficiency is dependent on maintaining a predetermined air velocity through the burner mechanism. This precludes making significant changes to a given air make-up units delivered air volume without mechanically reconfirming the unit.
The attempts of placing a premix burner in the air stream in the early days of air replacement technology was short lived because the panel fans that were used were loud and could not handle the static loads of the supply plenum filters. Once the capability of the blower with the ability to handle additional static was introduced the most effective method for air replacement was established as the paradigm. Direct fired incline burners with associated profile plates that adjust the airflow across the burner proved to be the most efficient technology. These profile plates were first considered as fixed, as was the airflow through the unit. With changes in technology these profile plates have now become adjustable and the range of acceptable airflow across the burner has been increased.
In a traditional air make-up unit system, all of the replacement air is drawn across the burner assembly and any change in the delivered air volume will change the burner's air supply. This causes it to operate under less than optimum conditions. For this reason, traditional air make-up units are designed with fixed speed fans. Any reductions in delivered air volume are usually accomplished by partially closing a damper on the output of the air makeup unit (AM) to reduce the output volume or changing the speed of the blower with a variable frequency drive (VFD). A VFD adjusts the rotational speed of the fan motor to keep the ventilation system balanced. The cost to apply a VFD to control the motor for the unit with an in-line burner is significant because a larger motor size is needed for a system with an in-line burner due to the higher static load.
In an enclosed booth system, the air make-up unit discharges directly into the process within the booth. Based upon the assumption the air make-up unit operates at a fixed speed, the exhaust fan speed is varied as required to keep the booth balanced. Prior to the advent of electronic VFD units, a damper was placed in the exhaust stack. This damper added an adjustable static load to the fan. The damper was reduced as the overspray arresting filters loaded to maintain a constant air velocity through the booth. The range of actual air velocity changes under this or other schemes is limited.
Unfortunately, the resulting somewhat arbitrarily established, fixed volume air flow found in a typical spray booth system is not the optimum environment for efficiently applying a consistent high quality finish. Significant process economies, as well as improvements in the quality of the applied finish, can be achieved by reducing the air velocity in any given spray booth. A few astute finishing process owners have “tuned” their finishing process by adjusting the exhaust air volume of individual spray booths and making corresponding adjustments in the volume of replacement air delivered by the associated air make-up unit as well as the necessary mechanical changes to the air make-up units configuration. However, no one has designed an air make-up unit/spray booth system that can be mass produced, yet economically facilitate the tuning of individual installations to the precise needs of their respective finishing processes while simultaneously maintaining optimum air make-up unit operation.
In prior art spray booths, replacement air flow is either on or off because the effort required to vary its volume was complex and time consuming. The burner required a fixed combustion air velocity to achieve the necessary clean burn characteristics and therefore the overall replacement air volume couldn't be changed.